In recent years, a GaN-based compound semiconductor material has become of interest as a semiconductor material for use in short-wavelength light-emitting devices. Such a GaN-based compound semiconductor is formed on a substrate (e.g., an oxide single crystal such as a sapphire single crystal, or a Group III-V compound single crystal) through a technique such as metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE).
One characteristic feature of such GaN-based compound semiconductor materials is that the diffusion of current is small in a direction parallel to the light-emitting surface. Such poor current diffusion may be attributable to the presence of a large number of threading dislocations which exists through an epitaxial crystal from the bottom surface (substrate side) to the top surface. However, the reason has not yet been elucidated in detail. Meanwhile, a p-type GaN-based compound semiconductor has a resistivity higher than that of an n-type GaN-based compound semiconductor. Therefore, when a metal layer is stacked on a surface of a p-type GaN-based compound semiconductor layer, substantially no current diffusion occurs in the direction parallel to the p-type layer. Thus, when an LED structure is fabricated from a pn junction of such semiconductors, light emission is limited to only a portion under the positive electrode.
In order to overcome the above-mentioned drawback, a transparent positive electrode through which light emitted from a portion under the positive electrode is extracted is generally used. Specifically, in one proposed technique employed for commercial transparent products, a plurality of Ni layers and Au layers each having a thickness of some tens of nm are stacked on a p-type layer to form a stacked layer, and the layer is heated in an oxygen-containing atmosphere for alloying, thereby promoting reduction in resistance of the p-type layer and simultaneously forming a positive electrode having good transparency and Ohmic characteristics (see Japanese Patent No. 2803742).
The transparent electrode is produced from a material such as a conductive metal oxide or ultra-thin metal film. Direct bonding is difficult to perform with such a material or structure. Therefore, in general, a bonding pad electrode having a sufficient thickness is placed so that electric contact is established between the pad electrode and the transparent electrode. However, because of its relatively large thickness, the metal pad electrode does not exhibit transparency, and the light emitted from a portion under the pad electrode cannot be extracted to the outside, which constitutes a problem.
In a prior art structure to enhance adhesion of a pad electrode, a transparent electrode is partially cut, and a pad electrode is provided so as to bridge neighboring transparent electrodes, whereby bonding strength is enhanced by a portion in direct contact with a GaN semiconductor layer, and current diffusion is caused to occur in a portion being in contact with the transparent electrode (see Japanese Patent Application Laid-Open (kokai) No. 7-94782).
As described above, as the light emitted from a portion under the pad electrode cannot be extracted to the outside, there have been developed techniques for effectively utilizing current in which light emission is avoided in a portion under a pad electrode through inhibition of current injection into the portion.
Specifically, there have been disclosed some techniques for effectively attaining light emission in which injection of current into a portion under a pad is inhibited through provision of an insulating area under the pad electrode (see Japanese Patent Application Laid-Open (kokai) No. 8-250768 and No. 8-250769). There has also been disclosed a technique for inhibiting injection of current into a portion under a pad electrode in which the bottom-most layer of the pad electrode is formed from a metal having high specific contact resistance with respect to the p-type layer (see Japanese Patent Application Laid-Open (kokai) No. 10-242516).
However, studies carried out by the present inventors have revealed that employment of any of the above techniques reduces the ohmic-contact area of the positive electrode with respect to the p-type layer, thereby problematically elevating the drive voltage.